This invention relates to apparatus for polishing semiconductor or similar type materials, and more specifically to such apparatus which facilitates equalization of the downward pressure over the polished wafer surface and/or the polishing head of the apparatus.
Polishing an article to produce a surface which is highly reflective and damage free has application in many fields. A particularly good finish is required when polishing an article such as a wafer of semiconductor material in preparation for printing circuits on the wafer by an electron beam-lithographic or photolithographic process (hereinafter “lithography”). Flatness of the wafer surface on which circuits are to be printed is critical in order to maintain resolution of the lines, which can be as thin as 0.13 microns (5.1 microinches) or less. The need for a flat wafer surface, and in particular local flatness in discrete areas on the surface, is heightened when stepper lithographic processing is employed.
Flatness is quantified in terms of a global flatness variation parameter (for example, total thickness variation (“TTV”)) or in terms of a local site flatness variation parameter (e.g., Site Total Indicated Reading (“STIR”) or Site Focal Plane Deviation (“SFPD”)) as measured against a reference plane of the wafer (e.g., Site Best Fit Reference Plane). STIR is the sum of the maximum positive and negative deviations of the surface in a small area of the wafer from a reference plane, referred to as the “focal” plane. SFQR is a specific type of STIR measurement, as measured from the front side best fit reference plane. A more detailed discussion of the characterization of wafer flatness can be found in F. Shimura, Semiconductor Silicon Crystal Technology 191–195 (Academic Press 1989). Presently, flatness parameters of the polish surfaces of single side polished wafers are typically acceptable within a central portion of most wafers, but the flatness parameters become unacceptable near the edges of the wafers, as described below.
The construction of conventional polishing machines contributes to unacceptable flatness measurements near the wafer's edge. Polishing machines typically include an annular polishing pad mounted on a turntable for driven rotation about a vertical axis passing through the center of the pad. The wafers are fixedly mounted on pressure plates above the polishing pad and lowered into polishing engagement with the rotating polishing pad. A polishing slurry, typically including chemical polishing agents and abrasive particles, is applied to the pad for greater polishing interaction between the polishing pad and the wafer.
In order to achieve the degree of polishing needed, a substantial normal force presses the wafers into engagement with the pad. The coefficient of friction between the pad and wafer creates a significant lateral force on the wafer. This lateral force can give rise to certain distortions in the polish, such as by creating a vertical component of the frictional force at the leading edge of a wafer. The vertical component of the frictional force is created because the wafer is mounted to pivot about a gimbal point under influences of the lateral friction forces. A change in the net vertical force applied to the wafer locally changes the polishing pressure and the polishing rate of the wafer, giving rise to distortions in the polish. Often the uneven forces cause the wafer's peripheral edge margin to be slightly thinner than the majority of the wafer, rendering the edge margin of the wafer unusable for lithographic processing. This condition is a sub-species of the more general problems associated with wafer flatness, and will be referred to hereinafter as edge roll-off.
Improvements in wafer polishers have helped reduce edge roll-off. Recent designs have incorporated conic bearing assemblies between the wafer and the mechanism applying the polishing force while permitting free rotation of the wafer. Conic bearing assemblies are an improvement over traditional ball and socket configurations because the gimbal point of the mechanism is at a point below the bearing, nearer the interface between the wafer and the polishing pad. As the polishing pad rotates beneath the polishing head, friction between the pad and the wafer create horizontal forces on the head, creating a moment on the head. This moment cants the polishing head with respect to the pad, applying greater force to the leading edge of the head. By lowering the pivot point of the polishing head toward a work surface of the polishing pad, or slightly below the surface, the torque moment applied to the polishing head by frictional forces is either minimized, eliminated or imparted in a more desirable direction. Control of this moment results in more uniform polishing pressure at all points on the wafer and in more uniform wear of the polishing pad. Wafers polished with a gimbal point near the work surface exhibit superior flatness characteristics, particularly near the outer edge of the wafer where conventional polishing processes exhibit characteristic “roll-off” and near the center of the wafer where slurry starvation may occur. Roll-off occurs in polishers having a gimbal point above the work surface where the torque on the polishing head due to friction presses the leading edge of the polishing head, and the wafer, into the polishing pad. Slurry starvation occurs when the leading edge of the wafer and head press into the polishing pad, pushing the slurry forward and inhibiting the slurry from flowing between the pad and the wafer. Despite these improvements in the prior art, the edge of the wafer may still exhibit unacceptable roll-off and the center of the wafer may be insufficiently polished.
Controlling wafer rotation while lowering the gimbal point to at or below the work surface is more desirable, because controlling the gimbal point of the mechanism and the rotational speed of both the polishing pad and the wafer allows more control over the wafer polishing process. Freely rotating polishing heads, in contrast, provide little control over the polishing process, as the polishing head and wafer simply rotate in response to frictional forces between the wafer and the polishing pad. Frictional forces can change between wafers and from one polishing machine to the next (due to turntable and drive mechanism misalignment, for instance), varying the rotational speed of the polishing head and the characteristics of the wafer polish. This process can lead to uneven polishing between wafers and cause increased degradation of the interior of the polishing pad. Since a freely rotating wafer will tend to rotate at a faster rate, the inside of the polishing pad sees more linear feet of wafer, wearing the pad more quickly near the pad's center. When the pad wears more quickly near the center, wafer flatness degrades because the pad is no longer flat. If the rotational speed of the wafer is decreased, polishing quality is greatly improved due to more uniform wear across the polishing pad. Moreover, pad wear impacts any “dishing” or “doming” of the wafer surface, which can be more effectively controlled by the rotational speed of the wafer. Thus, an improved design is needed incorporating further features, such as a low gimbal point and wafer rotation control, for inhibiting edge roll-off and improving wafer flatness generally.